Riveting fasteners are commonly used in mass production applications. In applications where the rivet is also a fastener, such as an externally threaded bolt, anti-rotation or torque resistance means are typically provided, particularly in mass production applications. In automotive applications, for example, the riveted fastener must be able to withstand the torque of an air gun or pneumatic driver. The torque the fastener must withstand without loosening will, however, depend upon the particular application and the size of the fastener.
In a stud-type "PIERCEFORM" fastener, for example, as disclosed in U.S. Pat. No. 4,555,838, assigned to the assignee of the present application, an 8 mm bolt must be able to withstand 24 to 34 N.M. (Newton meters). A 10 mm bolt must be able to withstand a torque of 50 to 70 N.M. in automotive applications. 70 N.M. is equal to about 52 foot pounds. During assembly, the air gun or pneumatic driver is generally set at about 70 N.M. to threadably attach a nut on a 10 mm "PIERCEFORM" bolt. Therefore, if the bolt does not withstand a torque of 70 N.M., the self-riveting bolt will be twisted in the panel, destroying the assembly.
The self-riveting fasteners of the type described herein include a body portion and an integral tubular barrel portion having a circumference less than the body portion. The barrel portion is preferably driven into the panel as the barrel portion is riveted to the panel. In a stud-type "PIERCEFORM" fastener, the body portion preferably includes a radial surface or flange and a stud portion integrally joined to the body portion extending in coaxial alignment with the barrel portion, as disclosed in the above-referenced U.S. Pat. No. 4,555,838. More commonly, anti-rotation protrusions or nubs are provided either on the barrel portion, adjacent the body portion, or on the flange portion adjacent the barrel portion as disclosed, for example, in FIG. 1 of U.S. Pat. No. 4,810,143, also assigned to the assignee of the present application.
Although they are somewhat effective in resisting rotation, these anti-rotation protrusions or nubs create stress risers in the panel and reduce the thickness of the panel which may result in failure of the fastener and panel assembly, particularly under torque in mass production applications. As stated above, where the riveting fastener is a stud-type fastener, a nut is normally driven onto the threaded stud by a pneumatic driver. However, it has been found that anti-rotation protrusions or nubs result in inconsistent torque resistance of the fastener and panel assembly. This may be due to collapsing deformation of the nubs, the stress risers described above, thinning of the panel or incomplete deformation of the panel into the spaces between the nubs or protursions. As the nubs are driven into the panel, the wall thickness of the panel is reduced, resulting in cracks or stress risers. The fastener may loosen in assembly or during use under dynamic loads, particularly vibration. In an attempt to overcome the above-mentioned problems associated with anti-rotation nuts, channels have recently been used as anti-rotation means in nut-type fasteners.
The problem of providing secure anti-rotation means for self-riveting and self-piercing and riveting stud fasteners of the type described above remains, particularly in mass production applications of such fasteners where a nut fastener is driven onto the riveted fastener with a pneumatic driver. This problem has not been solved with torque resistance nubs or protrusions and therefore the need remains for a more secure torque-resistant mechanical interlock between the fastener and the panel.